Generally, the commercial value of alkylene oxide polymers increases with the degree of polymerization. For example, when an ethylene oxide polymer as described above is used as a pulp dispersant for paper manufacturing, the optimum amount of its use varies depending on the molecular weight thereof. Because the effect generally increases as the molecular weight of the polymer increases, a polymer with higher degree of polymerization makes it possible to accomplish the object with a smaller amount when compared to a polymer with lower degree of polymerization. In other words, the use of a polymer with low molecular weight is uneconomical because it results in an increase in the amount of a polymer used, and it is also undesirable for paper manufacturing because it worsens the homogeneity of paper surface. On the other hand, the use of a polymer with high degree of polymerization as a thickener offers not only the excellent viscosity increasing effect with a smaller amount of use but also a great viscosity increasing effect which cannot be achieved by a polymer with low degree of polymerization, because an increase of the degree of polymerization results in a rapid rise of aqueous solution viscosity.
For these reasons, a development of a novel method which permits industrially reproducible manufacturing of alkylene oxide polymers with higher degree of polymerization is in demand in the relevant field.
A large number of studies have been made on the polymerization of alkylene oxides, particularly on ethylene oxide and propylene oxide. For example, it is known that the product obtained by reacting an organic zinc compound with a polyhydric alcohol and further reacting the reaction product with a monohydric alcohol, or the product obtained by reacting an organic zinc compound with a monohydric alcohol and further reacting the reaction product with a polyhydric alcohol (Japanese Patent Examined Publication Nos. 7751/1970 and 27319/1978) exhibits excellent catalytic activity on alkylene oxide homopolymerization or copolymerization of two or more alkylene oxides so that a polymer with a high degree of polymerization can be obtained.
It has been pointed out, however, that these methods are faulty, i.e., industrially advantageous production cannot be achieved, because the reproducibility of polymerization velocity, the degree of polymerization of the polymer obtained, etc., are poor. Further, its yield is insufficient and the polymerization product becomes lumpy. Moreover, even the degree of polymerization remains unsatisfactory. Alternatively, it has been attempted in another method to obtain a polymer with high reproducibility by using as a catalyst the product obtained by reacting an organic zinc compound and a polyhydric alcohol (Japanese Patent Laid-Open No. 232433/1987) in contact with various fine granular metal oxides and nonionic surfactants as dispersion aids. However, this method has been pointed out to have difficulty in the catalyst preparation.